England's most prestigious universities will have to double the amount they spend on widening access to poorer students if they charge the maximum tuition fees, government guidance has warned.

MPs voted in December to raise fees from £3,350 a year to £6,000 in 2012, and up to £9,000 in "exceptional cases".

Universities that want to charge more than £6,000 will have to draw up an agreement with the Office for Fair Access (Offa) stating how they will widen their pool of applicants.

The warning came as Sir Martin Harris, director of Offa, said there was a "real risk" when fees rose that teenagers from low-income homes would believe they could not afford university.

The watchdog's guidance states that universities that wish to charge £9,000 but which have a student intake that fails to reflect the English population will be forced to more than double their efforts to widen access.

The universities of Exeter and Cambridge and Imperial College London have already said they intend to charge £9,000; Oxford has said it wants to charge at least £8,000. David Willetts, the universities minister, has called for a system in which institutions charge varying amounts.

Offa said that universities that had the least diverse student population but were the most academically selective spent an annual average of £400 a student on widening access; from next year, they will be asked to spend up to £900.

Its guidance advises universities to come up with targets for taking teenagers from low-income homes, and those in care. They will also be told to come up with targets for increasing their pool of candidates with disabilities, as well as those from under-represented ethnic-minority backgrounds.

Harris said progress in improving access to the most selective universities had remained "virtually flat" in the last few years. "Those charging the highest fees will need to spend significantly more on access measures than those charging lower fees," he said. Universities had to target the most disadvantaged "more tightly".

Universities will be free to choose how best to increase diversity, but they will be encouraged to pour money into outreach work in schools and colleges, rather than into bursaries and scholarships. These incentives have been found to have little effect, Offa said.

At the moment, 85% to 90% of some universities' access funds are spent on bursaries and scholarships. Universities should also consider waiving fees.

The watchdog advises institutions to employ staff to help teenagers make decisions on which subjects to study at A-level and which degree course to aim for.

Offa wants more universities to take students' backgrounds into account. This could mean assessing how well an applicant has performed at school in relation to how good their school is. Harris said "relatively few" universities did this; Oxford and Bristol are among those that do.

Universities with a more diverse student body will still be expected to spend more on outreach, but to concentrate on reducing the number of students who drop out of their courses.

If universities break their access agreements, the watchdog will be able to stop them charging fees above £6,000.

A university can demand a review of its access agreement from the Department for Business, Innovation and Skills if it cannot agree on it with the watchdog.

Last week, in a speech to the British Academy, Willetts said fees for arts and humanities students need not be more than £6,000 because these courses were cheaper to teach than sciences.

Universities argue that they have to charge higher fees to make up for government cuts of £940m over the next academic year.

Wendy Piatt, director of the Russell Group, which represents large, research-intensive universities, including Oxbridge, said prestigious universities were committed to broadening access. But she insisted the "most important" reason for the under-representation of poorer students at leading universities was that they were failing to achieve the required grades at school.

The lecturers' trade union, the University and College Union, warned that outreach work would fail to overcome the fears of students and their families from poor and average-income homes, who would be "very wary of getting themselves into thousands of pounds of debt".

Willetts said universities were being asked to work much harder to recruit students from disadvantaged backgrounds. "The government has been clear that universities charging more than £6,000 who are not meeting their access benchmarks should redouble their efforts to widen participation," he said.